1. Field of the Invention
The present invention relates to a method utilized in a wireless communication system, and more particularly, to a method of handling device-to-device communication in a wireless communication system and a communication device thereof.
2. Description of the Prior Art
A long-term evolution (LTE) system supporting the 3rd Generation Partnership Project (3GPP) Rel-8 standard and/or the 3GPP Rel-9 standard is developed by the 3GPP as a successor of a universal mobile telecommunication system (UMTS) for further enhancing performance of the UMTS to satisfy increasing needs of users. The LTE system includes a new radio interface and a new radio network architecture that provides high data rate, low latency, packet optimization, and improved system capacity and coverage. In the LTE system, a radio access network known as an evolved universal terrestrial radio access network (E-UTRAN) includes multiple evolved Node-Bs (eNBs) for communicating with multiple user equipments (UEs), and communicating with a core network including a mobility management entity (MME), a serving gateway, etc., for Non-Access Stratum (NAS) control.
A LTE-advanced (LTE-A) system, as its name implies, is an evolution of the LTE system. The LTE-A system targets faster switching between power states, improves performance at the coverage edge of an eNB, and includes advanced techniques such as carrier aggregation (CA), coordinated multipoint (CoMP) transmission/reception, uplink (UL) multiple-input multiple-output (MIMO), etc. For a UE and an eNB to communicate with each other in the LTE-A system, the UE and the eNB must support standards developed for the LTE-A system, such as the 3GPP Rel-10 standard or later versions.
Starting from 3GPP Rel-12, a feature is included to allow UEs to communicate with each other directly, which is referred to as device-to-device (D2D) communication or Proximity-based Services (ProSe) communication. The D2D communication finds its applications on areas including public safety and non-public-safety services that would be of interest to operators and users. Proximity-based applications and services represent an emerging social-technological trend. The introduction of the D2D communication capability in LTE would allow the 3GPP industry to serve this developing market, and will, at the same time, serve the urgent needs of several public safety communities that are jointly committed to LTE.
Please refer to FIG. 1, which is a schematic diagram of protocol stacks for D2D direct discovery and communication. The first UE may start D2D communication with the second UE by transmitting data to the second UE when the first UE detects that the second UE is in its vicinity by itself or via assistance from a network. However, the second UE may keep moving so that the second UE may not receive any data after the second UE leaves the maximum range where data from the first UE can reach. The first UE is not aware of this situation and may keep transmitting data to the second UE. This wastes battery power of the first UE. In addition, when the first UE finishes transmitting data, the second UE does not know that the first UE has finished the transmission. In such a condition, the second UE may keep monitoring the channel for receiving data from the first UE since the second UE thinks that the first UE may keep transmitting data. This wastes battery power of the second UE. Thus, there is a need for improvement over the prior art.